PENETRATING SPACE

Taranaki Daily News, 23 February 1935, Page 13 (Supplement)

PENETRATING SPACE

THE EARLIEST LENSES

(By tire

Rev. B. Dudley, F.R.A.S.)

Who first discovered the principle of magnification? is one of those questions which may never be satisfactorily answered. The subject is elaborated in the Journal of the Royal Astronomical Society of Canada by William W. Ford. Euclid in the third century, 8.C., he reminds us, investigated the laws of refraction, and Seneca at the beginning of the Christian era was well familiar with them, in idea at least. Towards the close of his life he wrote’ that “letters, however small and dim, are comparatively larger and distinct when seen through a glass globe filled with water.” It is not so easy to trace back to its origin the actual use of a lens for purposes of magnification, although there is good reason for the opinion that a reference to the employment of spectacles occurs in the writings of Bernard de Gordon of Montpelier in 1305. “The real development of the knowledge of magnification came from the work of the mathematicians and physicists, beginning with Euclid,” states the writer. Ptolemy in the middle of the second century, A.D., however, was first, it would seem, to seriously inquire into the laws of the refraction of light. The optical properties of curved mirrors were known to some Arabian investigators at the end of the 10th century or the beginning of the 11th. Roger Bacon was acquainted with the use of the simple lens and wrote about it, recommending the employment of lenses for defective sight. It was Leonardo da Vinci (1452-1519) who developed the camera obscura while about the same period Maurelico the mathematician made a thorough study of refraction produced by air, water and glass. Descartio in 1637 gave descriptions of simple lenses mounted on stands. Microscopes of somewhat crude form were made by Athanasius Kircher in 1671; these enlarging objects about 32 diameters. Leeuwenhoek, in Holland, developed the microscope somewhere about the end of the 17th century, and personally owned some hundreds of these instruments of one kind or another, and on his death bequeathed 26 simple microscopes, made from lenses ground by himself and set in silver, to the Royal Society of London. Some of his instruments were capable of magnifying 150 and 160 times. Mr. Ford states “the claim of Jan Lippershey to the honour of the discovery of the telescope rests primarily upon the Acts of the States-General preserved in the government archives at the Hague.” He was a spectacle maker of Middleburg and found that two lenses put together in a tube made distant objects look as if they were near. The military value of these “spy glasses” was quickly recognised. “The Jansens began to manufacture both types of instruments, fleaglasses or fly-glasses (microscopes) and spy-glasses (telescopes) in the early part of the seventeenth century.” ' It remained, however, for Galileo to discover the scientific use of the telescope. One writer thus describes Galileo’s procedure: “He fastened two - lenses at .the end of a leaden tube, one plaqo-concave, one plano-con-vex. Looking,through the concave eyepiece he observed that objects seen were three times nearer and nine .times larger than when observed with the naked eye.” It was not long before Galileo improved upon his first attempts to such extent that objects seen by telescopic power were a thousand times greater and thirty-fold nearer than if observed by the unaided vision. Visitors to the Galileo museum in Florence are shown examples of these instruments made by Galileo himself. There is some doubt whether Galileo discovered the microscope, although there is reason for the belief that he presented a microscope to the King of Poland in 1612. The astronomical telescope inverts objects looked at; but Galileo lost no time in making some which had erecting systems included in them. By the middle of the seventeenth century telescopes and microscopes began to be made in large numbers in Holland, improvements continually being effected in their construction. Telescopes are of. two kinds, refracting and reflecting, the former possessing the greater magnifying power in proportion to size. The magnification is proportional to the length of the instrument as well as to the shortness of the focus of its eyepiece. Thus the use of different eyepieces yields various magnifying powers; but those exceeding 1000 are rarely used, owing to the trembling of the earth’s atmosphere. The higher the magnification the greater the inconvenience arising from this cause. The object viewed looks as though it were “boiling”—the term often used by astronomers to describe this effect—so that, although there is an apparent expansion of the disc of a planet or other object being examined, there is a corresponding loss of detail and of visual steadiness. While the eyepiece does all the magnifying, it is to the object glass, or lens at the other end of the instrument, that we owe the light-grasping power. The greater the diameter of “the objective,” the more luminous is the object -under scrutiny. In the refracting telescope the rays of light fall upon the great lens and, passing through it, are converged to a focus at the eyepiece, where they are seen or photographed. Tije reflecting telescope is differently constructed. In place of a lens is a concave mirror, usually made of glass, upon which is spread a thin film of silver. This mirror or “speculum,” which lies in a cell at the bottom of the telescope, throws back the rays to the eyepiece into which the observer looks. Since the light-rays do not pass through the mirror, but are reflected from it, a less perfect glass will do the required work. Hence reflectors can be made of much greater diameter than refractors. For many kinds of celestial photography, especially in respect to faint objects, the reflector has distinct advantages over the refractor. The largest reflecting telescope in existence is the 100-inch diameter instrument at the Carnegie Institution Observatory, Mount Wilson, California. This reflector has won many triumphs and has materially assisted in enlarging our conceptions of the scale of the universe. The largest refractor is the one beneath the magnificent dome of the Yerkes Observatory, the diameter of the lens being 40 inches. It shows stars down to the 17th magnitude. The building, 326 feet in length, was designed by Dr. Ellery Hale. The floor, which is 75 feet across, weighs 374 tons, but is easily raised and lowered to suit the convenience of the observer at the instrument. This floor met with an accident soon after its construction in 1897, when it fell 50 feet to the ground. The molten glass for the new 200-inch reflector was poured very recently; but must remain untouched for 10 months in order to cool. A mishap occured to the first casting, last March; and so the firm made another, which has every promise of being a success.